This invention relates to a nuclear reactor having an emergency core cooling structure used to cool a core when an accident occurs in the nuclear reactor.
A conventional emergency core cooling structure has in general a driving means, such as a pump and a valve. Such an emergency core cooling structure requires to be provided with a plurality of independent systems taking an accident to an accessory machine into consideration. This causes the dimensions of the equipment to excessively increase. As disclosed in Japanese Patent Laid-Open No. 51395/1978, the techniques for simplifying the construction of an emergency core cooling structure have been proposed, in which the emergency cooling water stored in a tank which is provided in a reactor vessel is injected into a core in an emergency by utilizing a difference between the pressures in the tank and core. According to such techniques, only a check valve is required as a driving means but, in these techniques, no consideration is given to the cooling water storage capacity and the core cooling performance. A reactor vessel contains various kinds of in-pile structures, and has a limited space in which a tank can be installed. If the reactor vessel is enlarged so as to install a tank therein, the manufacturing cost increases. If a tank is installed in the reactor vessel, the cooling water in the tank thermally equilibrates the primary coolant in the reactor vessel, and the temperature of the cooling water in the tank becomes equal to that of the primary coolant. Consequently, when the cooling water in the tank is injected into the core, it is boiled under vacuum and supplied in the form of vapor to the core, so that the core cooling effect of the cooling water becomes small.